FIELD OF THE INVENTION
The invention lies in the electronics field. Specifically, the invention relates to a circuit having a transmitter for transmitting a transmission signal to a transducer and a receiver for receiving a reception signal from the transducer. In addition, the invention relates to a method for transmitting a transmission signal to a transducer and for receiving a reception signal from the transducer.
In particular, the invention also relates to a transmitting/receiving circuit, which is also referred to as a transceiver circuit, as well as a transmitting/receiving method for an ultrasonic transducer.
Ultrasonic transducers are usually operated in the so-called pulse echo method. Such transducers must accordingly be able to transmit and to receive alternately. An ultrasonic transducer of this type may be constructed from one or more piezoelectric individual elements arranged for example in a linear or else in a two-dimensional array. In the transmission mode, these individual elements are excited to produce high-frequency oscillations by means of electrical transmission signals which may, if appropriate, be offset in terms of their phase angle with respect to one another for each individual element. As a result, an ultrasonic pulse is generated and is radiated into an examination object to be imaged. The precise focus region of the ultrasonic pulse in the examination object can be varied by way of the driving of the piezoelectric individual elements of the ultrasonic transducer. The ultrasonic pulse is reflected within the examination object in the direction of the ultrasonic transducer. The reflection can occur for example at discontinuities in the examination medium such as e.g. interfaces between materials of different acoustic impedance. These echo pulses or reflected pulses are received by the transducer or the individual elements and converted into a corresponding electrical reception signal. That signal is fed to a receiver equipped with a very sensitive preamplifier in order to raise the signal level of the reception signal. The signal level is generally very low. The amplified reception signal can then be forwarded to a signal processor for evaluation of the information content and for generation of a pictorial representation.
In order to ensure efficient functioning, the transmission signal and the reception signal must be isolated from one another. Therefore, each transducer element is connected to a transmitting/receiving circuit which electrically connects the transducer element selectively either to the transmitter or to the receiver. A transmitting/receiving circuit of this type is also referred to as a transmit/receive separator.
The decoupling of the transmitter from the receiver is desirable in principle because the amplitudes of the transmitted and received signals differ very greatly from one another. A transducer element is typically excited with a signal level of more than 100 V. A reception signal reflected from the examination object, on the other hand, has a very low voltage amplitude in the mV range. The transmitting/receiving circuit decouples receiver and transmitter from one another in order to avoid damage to the sensitive preamplifier in the receiver caused by the powerful transmission signal during the transmission mode. In addition, the decoupling during the transmission mode prevents undesirable influencing of the transmission signal, for example in the form of distortion, by the receiver. On the other hand, decoupling of receiver and transmitter is also desirable during the reception mode, in order to keep transmitter noise away from the receiver.
Various transmitting/receiving circuits are known. The changeover and decoupling can be performed by a circuit either actively, that is to say under the control of a switching signal, or else passively, that is to say automatically. An active transmitting/receiving circuit normally contains a plurality of active components such as transistors which vary their properties in dependence on the switching signal present at a control input. However, an active transmitting/receiving circuit always requires a separate control line for each individual element of the ultrasonic transducer, via which control line the switching signal is transmitted. Moreover, an additional control circuit is required which synchronizes the changeover and decoupling with the transmission and reception signals. If synchronization fails, there is the risk that sensitive assemblies in the receiver will be destroyed. Passive implementations of a transmitting/receiving circuit are usually either limited in terms of their dynamic range or they comprise complicated components, such as, for example transformers with a plurality of windings and taps.
An earlier, commonly assigned German patent application DE 195 14 330 A1 discloses a transmitting/receiving circuit for an ultrasonic imaging system in which a transducer is optionally electrically connected to a transmitter in a transmission mode and to a receiver in a reception mode. For this purpose, the transmitting/receiving circuit contains at least one variable-capacitance diode connected in the reverse direction for the transmission signal of the transmitter between the transducer and receiver. Instead of a single variable-capacitance diode, it is also possible to provide a reverse-connected series circuit borne by two variable-capacitance diodes. When a variable-capacitance diode is used, however, it is possible for a high current pulse to be coupled into the receiver, particularly in the event of a steep rise in the transmission signal. This current pulse can then result in damage to the receiver.